Background

Growing awareness and environmental legislations are imposing restrictions on the emission of volatile organic compounds (VOCs). These restrictions have forced the coating manufacturer to formulate coatings comprising low or zero VOCs content. Emulsion polymerization is an important polymerization technique, as it yields high molecular weight polymers with low viscosity. Moreover, polymers can be tailor-made in order to exhibit desired composition and morphology [1–4].

Isopropenyl acetate (IPA) is one of the monomers from acetate family which has an unsaturation with a methyl group attached to the ethylenic carbon. It can be prepared by various methods using synthetic chemicals, but the synthesis from green resources such as waste molasses is a new and attractive field of monomer preparation. This monomer is therefore an eco-friendly and cost-effective option to the acrylic monomers which are widely used. The typical structure of IPA is shown in Figure 1. The unsaturation present in the monomer structure can be utilized for free radical polymerization by various techniques.

Figure 1
figure 1

Structure of isopropenyl acetate.

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IPA was first polymerized via peroxide catalyzed polymerization reaction in 1950 [5]. Until to date, copolymers of isopropenyl acetate with maleic anhydride by both bulk and solution polymerization technique have been reported. Its copolymers with vinyl chloride were also synthesized by Unruh et al. and Basche et al. [6, 7]. Copolymer of isopropenyl acetate with indene monomer prepared by radical polymerization at high pressure was also prepared by De et al. to study the relative reactivity ratios [8]. The poly (styrene-co-isopropenyl acetate) copolymer prepared by free radical polymerization at 70°C was used as macroinitiator for cationic polymerization of isobutylene to synthesize graft copolymers of styrene-co-isopropenyl acetate-graft-polyisobutylene [9]. However, its slightly lower reactivity with other acrylic and vinyl monomers is a concern for preparing its copolymers with high conversion [10].

IPA has not been studied much via emulsion polymer technique. In the current work, we report the emulsion polymerization using IPA along with other acrylic monomers to study the feasibility of using IPA as a co-monomer for polymer synthesis.

Methods

Materials

Isopropenyl acetate was supplied by Godavari Bio refineries Ltd, Mumbai. BA and MMA were procured from SD fine Chemical Ltd. and were purified by using 10% of NaOH solution followed by washing with de-ionized water to remove inhibitor and were dried over sodium sulfate. Sodium carbonate, potassium persulfate (KPS), and tert-butyl hydroperoxide (TBHP) were lab grade chemicals purchased from S.D. Fine Chemicals Ltd (Worli Road, Mumbai, India). Dowfax 2A1 obtained from The Dow Chemical Company (Mumbai, India) was used as surfactant. Deionized water was used throughout the experiment.

Preparation of emulsion polymers

To study the emulsion polymerization of IPA along with BA and MMA, emulsions with variable molar proportions of IPA were prepared. IPA content was varied from 0.2 to 1.0 mol with respect to each mole of BA and MMA. The emulsion formulations prepared also contained 0.5% radical initiator KPS (of total monomer on weight basis), anionic surfactant Dowfax 2A1 and sodium bicarbonate as a buffer. All the emulsions were formulated for 41.5% solid content. The emulsions with molar concentrations 0.2%, 0.4%, 0.6%, 0.8% and 1.0% of IPA were prepared. These are hereafter denoted as BMI20, BMI40, BMI60, BMI80 and BMI100 respectively; while the emulsion with 0% IPA is denoted as BM. The typical formulation for emulsion BMI100 is summarized in Table 1. Figure 2 is as schematic representation of terpolymer synthesis.

Table 1 Typical formulation for emulsion BMI 100 (with 1:1:1 mole ratio of IPA/BA/MMA)
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Figure 2
figure 2

Structure of BA/MMA/IPA terpolymer.

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Characterization

The percentage conversion of monomers and nonvolatile matter (NVM) of emulsion polymers were determined by gravimetric method. Malvern Zetasizer instrument (Malvern Instruments Ltd., Malvern, Worcestershire, UK) was used to measure zeta potential (ζ-potential) of emulsions. Particle size of emulsions was determined by Malvern Instruments Ltd. (Mastersizer 2000 Ver. 5.30.010). Viscosities of the emulsions were measured by using Brookfield viscometer (Antonpar, GMBH, Austria) using spindle no.1 at 20 rpm. Molecular weight determination of emulsions was done using GPC by Waters 2414-refractive index detector (Waters Corporation, Milford, MA, USA) with flow rate of 1 ml/min in THF as mobile phase.

Fourier transform infrared (FTIR) spectrometric analysis of polymer films was done using Perkin Elmer instrument (PerkinElmer Inc., Waltham, MA, USA) to analyze the chemical structures of the emulsion polymer. Differential scanning calorimetry (DSC) was carried out to determine glass transition temperatures of the emulsion polymer films using TA instrument Q100 DSC (TA Instruments, New Castle, DE, USA). All the samples were analyzed for temperature range of −40°C to 50°C at the heating rate of 10°C/min under nitrogen as a purge gas with a flow rate of 40 ml/min. TGA was conducted to the thermal changes in the emulsion polymer films using Q100 DSC. The films were heated from 25°C to 500°C at the rate of 10°C/min under 40 ml/min nitrogen flow. The tensile strength and percent elongation of the emulsion films were evaluated by using Universal Testing Machine LR-50 K, Lloyd Instrument, West Sussex, UK with 500 N load cell. For measurement of tensile strength, the emulsion films were casted on Teflon sheet and were allowed to dry at room temperature. The films were cut (1 × 10 cm dimension) and conditioned prior to evaluation.

Results and discussion

The aim of this work was to study the feasibility of using isopropenyl acetate as a monomer for the preparation of emulsion polymers. Emulsion polymers were prepared from IPA, butyl acrylate (BA), and methyl methacrylate (MMA). IPA was charged at variable molar concentration with respect to that of BA and MMA. The synthesized emulsion polymers were further evaluated for incorporation of IPA in the polymer chain and to study its effect on the emulsion properties.

Polymerization conversion

Figures 3 and 4 show nonvolatile matter of emulsion polymers and polymerization conversion. It was observed that the percentage conversion of the prepared emulsions and conversion of IPA decreases as the proportion of IPA in the monomer feed is increased from 0.2 to 1 mol. Conversion of IPA was calculated by excluding proportion of BA and MMA from total polymer content by assuming 98% conversion of these monomers based on the conversion in BM polymer. The overall conversion was also observed to decrease in the same order. For every 20% increase in the molar concentration of IPA, the polymer conversion was observed to decrease by up to 5% from polymer BMI20 to polymer BMI100. This could be due to the lower reactivity of IPA during polymerization. IPA is reported to show degradative chain-transfer during polymerization. During polymerization process, IPA forms a free radical after elimination of hydrogen atom. The free radical formed is further stabilized by resonance (Figure 5) which results in its limited or no polymerization [11].

Figure 3
figure 3

Solid content of respective emulsion.

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Figure 4
figure 4

Polymerization conversion and conversion of IPA.

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Figure 5
figure 5

Resonance stabilization of monomer free radical.

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Particle size and zeta potential

The average particle size for all emulsions was found to be in the range of 0.165 to 0.168 μm. The stabilization of these systems was provided by an anionic surfactant (Dowfax 2A1). The electrostatic stability provided by the surfactant is related to the ζ-potential. Emulsions with the ζ-potential values lower than −35 mV are considered as optimum for negatively charged latex and higher than +35 mV for positively charged system [8, 9]. The present emulsions having negatively charged latex exhibiting ζ-potential values less than −35 mV were observed to be stable indicating that the electrostatic interactions were enough to stabilize the emulsions.

Viscosity

All the emulsions were evaluated for viscosity by Brookfield viscometer using spindle no.1 at the speed of 20 rpm and the viscosities for all the samples were observed to be in the range of 0.93 to 0.95 cP at 25°C.

Molecular weight

The emulsions were also characterized by gel permeation chromatography (GPC) to evaluate their molecular weight (Mn). It was observed that the molecular weight of all the emulsion polymers was more than 200,000, and polydispersity was in the range of 1.01 to 1.03.

Fourier transform infrared spectroscopy

The prepared emulsion films were further analyzed by Fourier transform infrared spectroscopy (FTIR). Figure 6 shows FTIR spectrum of polymer BMI60. The transmission band at 3,015 and 2,969 cm−1 are the characteristic stretching peaks of C-H (CH3, CH2); band at 1,737 cm−1 represents stretching vibration peak of C=O; 1,440 cm−1 represents distortion vibration of -COO-; 1,365 cm−1 shows rocking vibration of CH3. The stretching vibration of C=C disappeared within the range 1,640to 1,700 cm−1 which confirms the conversion of monomers into polymer.

Figure 6
figure 6

FTIR spectrum of BA/MMA/IPA terpolymer.

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Differential scanning calorimetry

Figure 7 shows the glass transition temperature (Tg) of emulsion polymers. The Tg value for the terpolymer of IPA, BA, and MMA was observed to be higher than that of copolymer of BA and MMA in all cases. The Tg of homopolymer of IPA (PIPA) is reported to be 45°C [10] which is higher than the Tg of homopolymer of BA (−56°C) and lower than that of PMMA (105°C). The glass transition temperature for these terpolymers suggests that IPA was incorporated in the polymer backbone resulting in higher Tg than that of copolymer due to hard segment of IPA. However, there was no significant change in Tg values of terpolymers to confirm the actual proportion of IPA converted in the polymer. The observations suggest that beyond certain proportion of IPA in the monomer feed, the capacity to undergo polymerization reduces drastically for the IPA monomer and gives limited participation in polymerization process.

Figure 7
figure 7

DSC curves of the emulsion films.

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Thermo gravimetric analysis

Thermo-gravimetric analysis (TGA) was carried out to investigate the thermal stability of the polymer film. The effect of IPA on the thermal stability of the emulsion polymers was evaluated by comparing the thermal stability of copolymer with that of terpolymers. The TGA graphs for all the samples are as shown in Figure 8. The thermal stability of terpolymers was observed to decrease significantly, as the amount of IPA in the monomer feed was increased. This could be due to the fact that IPA undergoes two-stage decomposition under programmed heating. In the first stage, acetic acid is eliminated via a zipper deactivation mechanism as shown in Figure 9. The second stage generally occurs at higher temperatures which involves fragmentation of the polymer backbone and the formation of cold ring fraction. Figure 10 shows typical degradation of copolymer of IPA/MMA at higher temperature [12, 13]. This justifies the discussion made about the decreasing trend observed in the thermal stability of terpolymer.

Figure 8
figure 8

TGA curves of the emulsion films.

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Figure 9
figure 9

Degradation of IPA in polymer.

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Figure 10
figure 10

Typical degradation of IPA/MMA copolymer.

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It was observed that in case of polymer BM, 10% degradation occurred at 332°C whereas polymer BMI20 is degraded by 10% at 309°C. Furthermore, it was observed that the increase in the concentration of IPA in the emulsion feed resulted in faster degradation of the polymer as shown in Table 2. From the results, polymer BM was observed to be thermally more stable than terpolymers, and the thermal stability of emulsion terpolymer decreased with the increasing proportion of IPA in polymer.

Table 2 Temperature at which polymers degrade by initial 10%
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Tensile strength and percent elongation

The mechanical strength of polymer films was evaluated by measuring tensile strength and percent elongation using Universal Testing Machine. Table 3 shows results of mechanical strength. It was observed that the terpolymer emulsion films showed higher tensile strength and less percent elongation than that of the copolymer films. This could be due to incorporation of harder segment of IPA in backbone of terpolymer as against BA and MMA in the copolymer. While in the case of terpolymer, tensile strength was observed to increase up to 0.6 M of IPA in polymer BMI60 which further decreased in polymer BMI80 and polymer BMI100. This could be due to the lower reactivity of IPA after 60 mol% concentration in the monomer feed, resulting in higher proportion of softer segments of BA compared to that in polymer BMI20, BMI40 and BMI60 and thus increasing the overall flexibility and elongation of the films. The similar trend was observed in percentage elongation of the films. It decreased up to 0.1 M of IPA in polymer BMI60 and again increased in polymer BMI80 and polymer BMI100 with increasing concentration of IPA in the polymer backbone.

Table 3 Results of mechanical properties
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Experimental

Preparation of pre-emulsion

Pre-emulsion was prepared using calculated amount of BA and MMA with 40% of required KPS, 75% of required surfactant and 60% of required water. The above solution was mixed under magnetic stirrer until a stable pre-emulsion was formed. Ten percent of this pre-emulsion was taken in a four-necked flask to start the polymerization. The calculated amount of IPA was added in the remaining pre-emulsion which was then added to the reaction flask dropwise for duration of 3.5 h. The same procedure was followed for all the emulsion formulations.

Emulsion polymerization

All the emulsions were prepared by seeded emulsion polymerization technique. The polymerization was carried out in a 250-ml four-necked flask equipped with reflux condenser, mechanical stirrer, dropping funnels, and temperature controller. Nitrogen atmosphere was maintained throughout the reaction. The flask was heated over a water bath. The remaining quantities of initiator, surfactant, buffer, and water were directly added into the reactor along with 10% of total pre-emulsion prepared. The reactor was heated to constant temperature of 70°C with a stirring rate of 300 rpm until slight blue color appeared. The reaction temperature was then raised to 78-80°C and the remaining pre-emulsion was fed drop by drop into the flask for a period of 3.5 h. The reaction was further continued for 30 min after the addition of pre-emulsion was over. A digestive catalyst TBHP was added to take care of the unreacted monomer as it blocks the double bond present on the unreacted monomer so that it cannot lead to any further side reaction. The reaction was continued for another 10 min. The reaction temperature was then reduced to room temperature and the emulsion was filtered. All the emulsion polymers were synthesized following the aforesaid procedure under identical conditions.

Conclusions

Emulsion polymers using isopropenyl acetate as a co-monomer were successfully prepared by seeded emulsion polymerization. The reactivity of IPA towards copolymerization was observed to reduce with increasing proportion of IPA in the monomer feed. This also affected the overall polymer conversion which could be due to the hindrance provided by the stabilized IPA radicals. The consistency and particle size of emulsions were unaltered in all cases. Incorporation of IPA in the terpolymer showed higher glass transition temperature than those of BA-MMA copolymer, suggesting the presence of adequate quantity of IPA in the polymer backbone. However, the terpolymer emulsion showed lower thermal stability than those of BA-MMA copolymer emulsion due to possible two-stage degradation of IPA at higher temperature. Terpolymer showed higher tensile strength and lower percentage elongation than that of copolymer; but in the case of terpolymer, increase in tensile strength was observed only up to polymer BHI60 but beyond that, it was observed to decrease in polymer BHI80 and polymer BHI100, exhibiting 0.6 M of IPA in the monomer feed as the optimized one under the scope of this work.

In summary, IPA is a low-cost radical monomer and could be a partial replacement for various acrylic monomers after successful study for the improvement in its polymerization mechanisms, reaction parameters, kinetics of the reaction and conversion.